Thermal barrier coatings which are applied to components are known from the field of gas turbines, as described for example in EP 1 029 115.
Thermal barrier coatings enable components to be used at higher temperatures than those permitted by the base material, or allow the service life to be extended.
Known base materials (substrates) for gas turbines allow temperatures of use of at most 1000° C. to 1100° C., whereas a coating with a thermal barrier coating allows temperatures of use of up to 1350° C.
The temperatures of use of components in a steam turbine are much lower, and consequently these demands are not imposed in this application.
It is known from EP 1 029 104 A to apply a ceramic erosion-resistant layer to a ceramic thermal barrier coating of a gas turbine blade or vane.
It is known from DE 195 35 227 A1 to provide a thermal barrier coating in a steam turbine in order to allow the use of materials which have worse mechanical properties but are less expensive for the substrate to which the thermal barrier coating is applied.
U.S. Pat. No. 5,350,599 discloses an erosion-resistant ceramic thermal barrier coating.
US 2003/0152814 A1 discloses a thermal barrier coating system comprising a substrate made from a superalloy, an aluminum oxide layer on the substrate and a ceramic as outer ceramic thermal barrier coating.
EP 0 783 043 A1 discloses an erosion-resistant layer consisting of aluminum oxide or silicon carbide on a ceramic thermal barrier coating.
U.S. Pat. No. 5,683,226 discloses a component of a steam turbine with improved resistance to erosion.
U.S. Pat. No. 4,405,284 discloses an outer metallic layer which is considerably more porous than the underlying ceramic thermal barrier coating.
In its discussion of the prior art, EP 0 783 043 A1 discloses the formation of an erosion-resistant coating in two layers, specifically comprising an inner metallic layer and an outer ceramic layer.
U.S. Pat. No. 5,740,515 discloses a ceramic thermal barrier coating to which an outer, hard ceramic silicide coating has been applied.
WO 00/70190 discloses a component wherein an outer metallic layer is applied, this layer containing aluminum in order to increase the oxidation resistance of the component.
The thermal barrier coating is strongly eroded on account of impurities in a medium and/or high flow velocities of the flowing medium which flows past components having a thermal barrier coating.